Fabrication of α-alumina by a combination of a hydrothermal process and a seeding technique

Abstract

This paper describes a method for producing [Formula: see text]-Al2O3 at low temperatures by a combination of a hydrothermal process and a seeding technique. White aluminum hydroxide precipitate was prepared by a homogeneous precipitation method using aluminum nitrate and urea in aqueous solution. Peptization of the precipitate by acetic acid at room temperature transformed it into a transparent alumina sol. The alumina sol was treated with a hydrothermal process. [Formula: see text]-Al2O3 particles serving as seeds were added to the hydrothermally treated alumina sol. The sol containing the [Formula: see text]-Al2O3 particles was transformed into an [Formula: see text]-Al2O3-seeded alumina gel by drying at room temperature. The non-seeded alumina gel was amorphous or showed fine crystallites and began to crystallize into [Formula: see text]-Al2O3 at 900[Formula: see text]C. The [Formula: see text]-Al2O3-seeding promoted the crystallization of the alumina gel to [Formula: see text]-Al2O3. A remarkable [Formula: see text]-Al2O3 crystallinity was achieved with an increase in [Formula: see text]-Al2O3 particle content by weight in the final seeded alumina gel. For an [Formula: see text]-Al2O3 particle content of 5%, the seeded alumina gel was partially crystallized to [Formula: see text]-Al2O3 by annealing at temperatures as low as 600[Formula: see text]C.

Towards Macroporous α-Al2O3—Routes, Possibilities and Limitations

“…Even more recently, Yamamura et al reported a hydrothermal method combined with a seeding technique, showing α-Al 2 O 3 reflexes in XRD at temperatures as low as 400 • C, yet without reaching complete conversion to α-Al 2 O 3 even for 5 wt.% seeding crystals [37]. A second publication shortly after shows XRD reflexes for α-Al 2 O 3 starting from 600 • C [38]. However, complete conversion to α-Al 2 O 3 is still not achieved until annealing temperatures reach at least 900 • C. The obtained particle sizes are in the sub-micrometer range, whereof relatively large specific surface areas may be inferred, although numbers are not presented.…”

Preparation of spherical α‐Al2O3 nanoparticles by microwave hydrothermal synthesis and addition of nano‐Al seeds

“…Reducing the activation energy barrier of crystalline transformation by adding crystalline seeds during calcination, hence lowering the calcination temperature, has proven to be an effective method for the preparation of α-Al 2 O 3 particles at low temperature. [21][22][23][24][25][26] Kobayashi et al 22 added a small amount of α-Al 2 O 3 seeds to alumina sol and obtained α-Al 2 O 3 particles at less than 1100 • C. Yamamura et al 23 combined hydrothermal and sol-gel methods with α-Al 2 O 3 as seeds, and even reduced the crystallization temperature of α-Al 2 O 3 to 900 • C, but the obtained α-Al 2 O 3 contained some transition-phase alumina. Except for the self-seeding with α-Al 2 O 3 as seeds, some other crystallized materials such as γ-Al 2 O 3 , δ-Al 2 O 3 , and α-Fe 2 O 3 are also used as seed materials.…”

“…Reducing the activation energy barrier of crystalline transformation by adding crystalline seeds during calcination, hence lowering the calcination temperature, has proven to be an effective method for the preparation of α‐Al 2 O 3 particles at low temperature 21–26 . Kobayashi et al 22 .…”

“…added a small amount of α‐Al 2 O 3 seeds to alumina sol and obtained α‐Al 2 O 3 particles at less than 1100°C. Yamamura et al 23 . combined hydrothermal and sol–gel methods with α‐Al 2 O 3 as seeds, and even reduced the crystallization temperature of α‐Al 2 O 3 to 900°C, but the obtained α‐Al 2 O 3 contained some transition‐phase alumina.…”

Sol-gel synthesis of α-Al2O3 with enhanced porosity via dicarboxylic acid templating

“…Macroporous α-Al 2 O 3 can be obtained via different routes, e.g., transformation of diaspore 6,7 , hydrothermal methods 8,9 , or anodic oxidation of aluminum chips and their successive calcination to porous alumina membranes 10,11 . Yet one of the most versatile methods is found in the sol-gel synthesis in presence of templates, starting from aluminum alkoxides 12 or aluminum salts 13–15 .…”